Acetylene is also indicated, however, with less sensitivity. Petroleum hydrocarbons, benzene, halogenated hydrocarbons and hydrogen sulfide are retained in the pre-layer. In the case of higher concentrations of interfering hydrocarbons, use should be made of a carbon pre-tube (CH 24 101). Higher concentrations of easily cleavable halogenated hydrocarbons (e.g. trichloroethylene), are liable to form chromyl chloride in the pre-layer which changes the indicating layer to a yellowish-brown. CO determination is impossible in the case of high olefin concentrations.